This includes:
- using the `std::fs::read_to_string` function,
- using the `lines` method on a `BufReader`,
- iterators directly rather than collecting everything in to a vec,
- adding a couple of `debug_assert`s,
- a little bit of general formatting (mixed tabs -> all spaces, correct
  indentation levels etc).

`man os-release` specifies that `/usr/lib/os-release` should be used if
`/etc/os-release` can't.